1. Field of the Invention
The present invention relates generally to the field of spinal fixation devices. More specifically, the present invention is directed to a method and apparatus for spinal fixation which incorporate a polydirectional hook for engaging the spine.
2. Description of the Related Art
The human spinal column includes more than twenty discrete bones. These bones are generally similar in shape. Despite their similar shape, however, they do vary substantially in size in accordance with their individual position along the spinal column. The bones are anatomically categorized as being members of one of three classifications: cervical, thoracic, or lumbar. The cervical portion of the spinal column, which comprises the top of the spine up to the base of the skull, includes the first seven vertebrae. The intermediate twelve bones are thoracic vertebrae. The remaining five bones are the lumbar vertebrae.
With reference to FIGS. 1 and 2, a portion of the human spinal column A is shown. The spinal cord B is housed in a central canal and protected from the posterior side by a shell of bone called the lamina C. Each of the lamina C have three large protrusions. Two of the protrusions extend laterally from the side ends thereof and are referred to as the transverse processes D. The third protrusion extends back and down from the center of the lamina C and is called the spinous process E.
The anterior portion of the spine includes a set of generally cylindrically shaped bones stacked one on top of the other which are referred to as the vertebral bodies F. The vertebral bodies are separated from one another by cartilage spacers referred to as intervertebral discs G. Bone bridges referred to as pedicles H couple the anterior vertebral body F to the corresponding lamina C and posterior elements D and E.
The spinal column is a highly complex structure which houses and protects critical elements of the nervous system. In spite of these complexities, the spinal column is a highly flexible structure, capable of a high degree of curvature and twist through a wide range motion. Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or threaten the critical elements of the nervous system housed within the spinal column.
A variety of systems have been disclosed in the art which achieve immobilization of portions of the spinal column by implanting artificial assemblies in or on the spinal column. These assemblies may be generally classified as anterior, posterior or lateral implants. Posterior implants are attached to the back of the spinal column generally by coupling to the pedicles with screws, or through hooks which attach under the lamina. In either case, the implants generally include elongate support rod elements which are coupled to the screws or hooks to immobilize several sequential vertebrae, for example to hold them stable so that adjacent bones may be fused with bone graft.
Such hook and rod assemblies generally comprise a plurality of hooks having rounded blade portions which are inserted posteriorly under the lamina between the transverse process and the spinous process. Difficulty may be encountered with the insertion of hooks under sequential lamina. In this regard, fixed hooks of the prior art are not able to self align in the sagittal and coronal planes. Correction of this difficulty requires time consuming reshaping of the rods or links used to connect the hooks. Such bending is a tedious process, which is inconsistent and adds unwanted time to an operation. In the event that the hooks are not securely fastened to the lamina, relative motion at the bone interface may lead to pseudoarthrosis or other ailments.
It is often the case that the failure of a hook is related to improper throat diameter. It is desirable to have lamina hooks offer the ability to adjust the throat diameter by changing the angle of the hook body to the rod or link. An adjustable throat diameter facilitates fixation on various locations of the spine and different size patients.
To a limited extent, it is also known to employ spinal fixation systems having a hook device adjustably carried by a rod. For example, U.S. Pat. No. 5,578,033 discloses a rod receiving body mounted to a blade portion such that the body may be maneuvered relative to the placement of the blade. The blade portion of the disclosed device has a semi-spherical head portion which sits above the lamina. The rod receiving portion is a separate coupling arrangement having lower, intermediate, and upper portions. The lower portion is slotted and tapered, and has a semi-spherical interior chamber in which the semi-spherical head is disposed. The intermediate portion has a side recess for receiving the rod. The top portion is threaded for a top locking nut. A locking ring is disposed about the coupling arrangement. A rod retaining sleeve is provided about the coupling arrangement. The sleeve has a lower surface which seats against the top of the rod.
While the device disclosed by U.S. Pat No. 5,578,033 may be suitable for certain applications, it is associated with certain disadvantages. For example, the clamping force for arresting movement of the hook relative to the rod is indirectly applied in an inefficient manner. In this regard, when the rod is in place, the locking nut is tightened to provide a downward force onto the sleeve. The sleeve, in turn, applies a force against the rod, which causes the locking ring to descend down the tapered lower portion, therein locking the semi-spherical head in the interior chamber of the coupling arrangement, and locking it into position relative to the blade portion. In addition, the adjustable hook disclosed by U.S. Pat. No. 5,578,033 is not adapted to be used with a plate or link system. The present invention is specifically intended to overcome these specific disadvantages, among others, associated with devices heretofore proposed.
According to one aspect, the present invention relates to an improved apparatus for spinal fixation which includes a hook member and a connector member such as a plate, link or rod which can be used for spinal stabilization.
An advantage of the present invention is to provide a method and apparatus for spinal fixation that incorporate a connector member and an improved polydirectional spinal hook.
Another advantage of the present invention is to provide a method and apparatus for spinal fixation that allows for hook alignment in both the sagittal and coronal planes without the need to bend the longitudinal member.
Another advantage of the present invention is to provide a method and apparatus for spinal fixation which offers the ability to adjust the throat diameter of a hook by changing the angle of the hook body, to thereby facilitate fixation on various locations of the spine and different size patients.
It is another advantage of the present invention to provide a polydirectional hook assembly for spinal fixation which more efficiently applies a clamping force for arresting relative rotation between a connector member and the hook assembly.